This invention relates to a coating composition for a plastics substrate.
Plastics substrates are generally impervious to materials such as printing inks. Thus, they tend to be unsuitable for direct use in ink-based printing processes because the ink has a tendency to remain at the surface of the substrate, without being absorbed.
It is well known to modify the surface characteristics of a plastics substrate by applying an absorbent coating composition to the substrate""s surface. The coating composition comprises an absorbent filler, which renders the substrate more absorbent to ink, and a polymeric binder, which binds the filler to the substrate""s surface.
Both water-based and solvent-based coating compositions are known. Solvent-based coating compositions generally show better adhesion properties to plastics substrates than water-based compositions. However, water-based compositions are preferred for environmental reasons, even though they can be picked or soaked off relatively easily. This problem may be alleviated by increasing the ratio of binder to filler in the coating composition. However, this reduces the absorbency of the coating, thereby reducing its ink retention properties. The water-sensitivity of water-based coatings is generally improved by including a cross-linking agent in the coating. This also reduces the tendency of such coatings to stick together, which is otherwise a problem when stacking sheets.
GB 2 177 413 describes the use of melamine formaldehyde and urea-formaldehyde resins as cross-linkers, to rigidify the coating composition and reduce its water-sensitivity. However, in view of the health concern over formaldehyde fumes, GB 2 177 413 proposes to replace the formaldehyde resins with a zirconium compound. The zirconium compound acts as cross-linking agent, and reacts with carboxyl groups present in the binder to render the binder substantially insoluble, and thus water-resistant. Suitable insolubilizing agents include aqueous zirconium compounds, such as ammonium zirconium carbonate (AZC).
Although the coating compositions of GB 2 177 413 work relatively well, it is desirable to improve the water resistance and adherence of these polymer coatings further.
Polyamide epichlorohydrin is known to be an effective cross-linking agent for cellulose paper. However, because it is a covalent compound, it has a tendency to retain static charge. These charges are difficult to dissipate and may cause overlying substrates to adhere to one another, making the substrates difficult to handle in subsequent processing operations (eg when feeding the substrate into a sheet feed printing process).